As for hardware resources of the VM or Virtualbox, you should be able to fit the base OS, all the dependencies, source, and build in a 30GB partition. For memory, 2GB should be considered the minimum. More memory will yeild better performance.

Install Developer Tools

As with the existing Windows 7 development steps, you'll need to install a number of prerequisites.

If you don't install the DirectX SDK, add the following to your .mozconfig:

ac_add_options --disable-webgl
ac_add_options --disable-gamepad

(optional) Windows SDK

Download and install the Windows 8 SDK from http://msdn.microsoft.com/en-us/windows/hardware/hh852363. But the Windows SDK isn't technically required (the files in Visual Studio 2011 will suffice). Like Visual Studio, this is a web downloader, so the bulk of the downloading is performed while running the installer. The default install options should be sufficient.

Then, create your .mozconfig: (with mozilla-build >=1.9.x you can skip this step. It will generate automatically an object folder and a mozinfo.json file with autodetected options)

# This specifies the directory where
mk_add_options MOZ_OBJDIR=@TOPSRCDIR@/obj-ff
# Uncomment the following line if you want to build with debugging code enabled:
#ac_add_options --enable-debug
# uncomment the following if you did NOT install the DirectX SDK:
#ac_add_options --disable-webgl
#ac_add_options --disable-angle
#ac_add_options --disable-gamepad

Then, you should be able to build and run Firefox by running the following commands in the mozilla-central directory. Building for the first time may take an hour or more, depending on your hardware!

./mach build
./mach run

Compiling Firefox for Windows 8 Touch ("Metro style")

To build Firefox with the Windows 8 "Metro" style interface enabled, you must install the Windows 8 SDK and the DirectX SDK. If you are using Visual Studio 2010 or any version of Visual Studio Express, you may need to install additional libraries. For details, see the Windows 8 Integration wiki page.

To enable the Metro feature, add the following line to your .mozconfig:

ac_add_options --enable-metro

Then build and run Firefox as above, and use the "Relaunch in Windows 8 Style" button in the menu to switch to the "Metro" interface.

Since WIndows 8 isn't officially supported yet, there may be a breakage. But the tree has been known to compile. If you run into errors, look for bugs here. If you see an issue not reported, please file a bug!